This invention relates to the release from aircraft of stores containing explosives which are initiated by some means external to the aircraft, i.e. impact, barometric pressure and so forth. It is essential that such stores should be kept in a safe, unarmed state during their preliminary handling, carriage and ultimate release and ejection to a pre-set distance from the aircraft. For this purpose, each store must include an arming pin which, in one position ensures that the contents of the store are in a safe condition and in an alternative position "arms" the store and thus allows the contained explosive to be initiated. The requirement is, of course, that the pin should remain in the "safe" position until such time as the store has been released from the aircraft and has moved away from the aircraft by a pre-set distance; in some cases it may be required that the pin remains in the "safe" position throughout the subsequent fall of the store.
In operation, therefore, the pin is moved to the "armed" position so as to arm the store when it reaches the pre-set distance from the aircraft and mechanical methods for this purpose essentially involve connecting the arming pin on the store to an arming unit on the aircraft through an arming wire, lanyard, or shear wire of specified length and shear load. One end of the wire is securely attached to the arming pin in the store and the other end is attached to a ring which is inserted into the arming unit on the aircraft and retained during flight.
The arming wire includes a point of weakness, e.g. a shear pin or weak-link of which the shearing force is greater than the force necessary to set the arming pin on the store. Accordingly, when the store is to be released in the armed condition, the ring of the arming wire is retained by the arming unit on the aircraft and when, after release, the store reaches the full extent of the arming wire, the arming pin on the store is set by the tension in the wire, immediately after which the wire shears and the store falls away in the armed condition. The major part of the arming wire remains with the store and proof that arming was selected and functioned correctly on release is the retention in the arming unit of that part of the arming wire in-board of the point of weakness.
If the store is to be released in a safe or un-armed condition, the ring or loop at the end of the arming wire must be released before sufficient tension is developed in the arming wire to set the arming pin. The arming unit on the aircraft must therefore retain the loop or ring with a force which is appreciably less than that required to set the arming pin on the store, but sufficiently large to prevent the wire from being dislodged by in-flight aerodynamic forces or ground crew manual loading forces such as are encountered during routine maintenance. The "safe" setting loading is generally of the order of 16 lbs (7 kg), which is significantly less than the force required to arm the store. The arming unit on the aircraft therefore usually includes a spring retaining device which is overcome when the tension in the arming wire exceeds the value just referred to and a positive lock which retains the ring or loop to arm the store as described above.
The disadvantage of such an arrangement is that although the spring retention force on the ring or loop is designed to be significantly less than that required to arm the store, the dynamic conditions of arming wire release can result in very high shock loads in the arming wire which may be sufficient to arm the store when not intended. In other words, a store which is intended to be released in a safe condition is then inadvertently armed.